1. Field of the Invention
The present invention relates to a method for hydrofluoric acid digestion of silica/alumina matrix material for the production of silicon tetrafluoride, aluminum fluoride and other residual metal fluorides and oxides
2. Description of the Related Art Including Information Disclosed Under 37 CFR .sctn.1.97-1.99
Currently large quantities of silica/alumina (SiO.sub.2 /Al.sub.2 O.sub.3) matrix waste material is produced by a number of commercial industries including, but not limited to, the spent fluid cracking catalyst unit (FCCU) materials, e.g. silica/alumina zeolytes, from the petroleum industry and the coal combustion by-products (CCBP), e.g. fly ash, slags, or bottom ash, produced from the use of fossil fuels by the power generation industry. The present invention relates to hydrofluoric acid digestion of these silica/alumina matrix material for the production of useful material including silicon tetrafluoride (also called tetrafluorosilane, SiF.sub.4), aluminum fluoride (also called aluminum trifluoride, AlF.sub.3) and other residual metal fluorides and oxides.
Silicon tetraflouride is an important intermediate, useful for the production of valuable products, such as pure silica, silanes, pure silicon for solar cells, silicon nitride for ceramic products, amorphous silicon for photo-voltaic cells, and etching medium for materials containing silicon in the semi conductor industry, and fluorinated carbo-silicon polymers for materials for architectural uses. Further, silicon tetrafluoride may be used to produce hydrogen fluoride and silicon dioxide. For example, U.S. Pat. No. 4,206,189 discloses a method wherein silicon tetrafluoride is subjected to water hydrolysis in the presence of sulfuric acid to produce hydrogen fluoride and silicon dioxide. Hydrogen fluoride is used in production of such important chemicals as organofluorine compounds (fluorinated plastics, freons, etc.), inorganic fluorides (cryolites, sodium and aluminum fluorides) for aluminum production, fluoride optical materials, oxidants for rocket engineering, fluoride inorganic compounds for atomic engineering, etc.. Silicon dioxide is used in the technical rubber industry as a filler for rubber stocks and as a raw material in the optical, radio engineering, electronic glassmaking, and other industries. It is the main component in compositions for manufacturing luminophores, sorbents for chromatography, etc.
Aluminum fluoride is useful as a precursor for making an oxide etchant for electronic applications and may also be useful as an ammonium source for diammonium phosphate.
In U.S. Pat. No. 4,615,872 a process for producing silicon tetrafluoride by hydrolysis of gases containing silicon fluoride is disclosed wherein the hydrolysate is reacted with sodium fluoride, potassium fluoride or barium fluoride and the reaction product obtained is decomposed thermally, thereby forming silicon tetrafluoride.
In U.S. Pat. No. 4,900,530 a process for the production of silicon tetrafluoride is disclosed starting from an aqueous solution of fluorosilicic acid that is reacted with concentrated sulfuric acid followed by a separating of the gaseous stream containing silicon tetrafluoride from a liquid stream of aqueous sulfuric acid. The gaseous stream is then washed with concentrated and cold sulfuric acid for the purpose of separating a gaseous stream of purified silicon tetrafluoride which may also be subjected to further purification process by means of techniques of absorption on an absorbent solid material.
In U.S. Pat. No. 4,470,959 silicon tetrafluoride gas is produced by reacting aqueous fluorosilicic acid and concentrated sulfuric acid in a vertical column.
In U.S. Pat. No. 3,961,030 an aluminum containing ore is treated with a fluorine acid such as hydrofluoric acid or fluorosilicic acid to produce aluminum fluoride which is recovered from the liquid phase of the reaction mixture by crystallization as AlF.sub.3 .multidot.3H.sub.2 O crystals which are then dried and dehydrated to yield aluminum fluoride which may be pyrohydrolyzed to produce alumina and hydrofluoric acid.
The disclosure herein is directed to a process for hydrofluoric acid digestion of silica/alumina matrix material for the production of silicon tetrafluoride, aluminum fluoride, and other heavy metals. This process is advantageous over the prior art in that it allows for the chemical digestion of a silicon/alumina matrix material which is of lower grade and often man-made such as spent fluid cracking catalyst unit (FCCU) materials and coal combustion by-products (CCBP) as opposed to naturally occurring high purity clays. Additionally, the initial chemical digestion of the silica/alumina matrix material is worked at ambient temperature and, although the exothermic nature of the initial reaction will heat the solution, the temperature remains under 100.degree. C. to prevent water vapor from being in admixture to the produced silicon tetrafluoride gas stream. Additionally, the silicon tetrafluoride gas stream is further purified by introducing the same through one or more, and preferably a series of, cold trap(s) to liquify or crystalize impurities out of the silicon tetrafluoride gas stream which may destroy or hamper electrical application of the final silicon metal.